In recent years, an optical disk such as a CD (Compact Disc) or DVD (Digital Versatile Disc) is being used more and more in many fields of audio, video, computers, and the like since it can record a large amount of information signals at high density. In particular, recently, the amount of data handled in a computer or the like such as moving picture information is dramatically increasing and, accordingly, the capacity of an optical disk is being increased by reducing the size of a recording pit and a track pitch.
In an optical recording medium such as an optical disk, at the time of reproducing signals recorded on the optical recording medium, a light beam has to be accurately applied to the track. It is therefore necessary to perform tracking control of making a light spot follow the track on which the signal is recorded.
Hitherto, a differential push pull (DPP) method is known as a representative detecting method. In the method, at the time of performing the tracking control, to cancel an offset of a tracking error signal indicative of an error in tracking which occurs in movement in the radial direction of the optical recording medium, the tracking error is detected.
In the DPP method, a light beam applied to an optical disk is split to a main beam and two sub-beams. The light beam is applied to an optical disk while shifting each of the sub-beams in the radial direction of the optical disk only by a “½” track pitch of a track in which the main beam is condensed. Reflection light from the optical disk is detected by a half-split detector. By using push pull signals including the sub-beams output from the half-split detector, a tracking error signal in which a push-pull offset is cancelled is detected. On the basis of the detected tracking error signal, tracking control is performed.
In particular, recently, as a method of suppressing fluctuations in the amplitude of a differential push-pull signal due to changes in the positions in the radial direction of the main beam and the sub-beams applied onto an optical disk, the following method is used. In the method, a diffracting grating is divided into four quadrants (regions) around the optical axis of a light beam as a center. Only in combinations of diagonal quadrants such as the first and third quadrants, lattice grooves are shifted only by a predetermined amount. Using the diffraction grating, a zeroth-order diffraction ray (that is, a main beam) and ± first-order diffraction rays (that is, sub-beams) to be applied to an optical disk are emitted. On the basis of reflection light of the emitted diffraction rays, a tracking error signal is detected.
The DPP method using the special diffraction grating gives effects similar to those of a normal diffraction grating to the zeroth-order diffraction ray as the main beam emitted from the light source and, on the other hand, produces effects, to the ± first-order diffraction rays, that a phase shift occurs due to a delay of the phase of the wavefront in a quadrant in which the lattice grooves are shifted from that of the wavefront in another quadrant.
Therefore, in the DPP method using the special diffraction grating, as described above, when the diffraction grating is divided into four quadrants (regions) around the optical axis of a light beam as a center and the lattice grooves are shifted only by a predetermined amount only in combinations of diagonal quadrants such as first and third quadrants, a push-pull signal generated in the quadrants (first and second quadrants) in the upper half of the sub-beam of ± first-order rays and a push-pull signal generated in the quadrants (third and fourth quadrants) in the lower half cancel out each other. In the whole light beam, fluctuations in the push-pull signal amplitude can be suppressed and a push-pull offset can be cancelled (for example, patent document 1).
Patent document 1: Japanese Patent Application Laid-Open No. 2001-250250